Inimer-containing random copolymers and crosslinked copolymer films for dense polymer brush growth

ABSTRACT

Crosslinkable random copolymers comprising atom transfer radical polymerization (ATRP) initiators and crosslinked copolymer films formed from the copolymers are provided. The random copolymers, which are polymerized from one or more alkyl halide functional inimers and one or more monomers having a crosslinkable functionality, are characterized by pendant ATRP initiating groups and pendant crosslinkable groups.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/771,922 that was filed Feb. 20, 2013, the entire contents ofwhich is hereby incorporated by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under 0832760 awarded bythe National Science Foundation. The government has certain rights inthe invention.

BACKGROUND

Polymer brushes are a broad class of materials consisting of a polymerchain tethered by one chain end to a surface. These brushes have avariety of applications especially in their ability to tune and modifysurface properties such as bioadhesion, wettability, and surfaceactivity. Two main methods for their preparation have emerged, namelygrafting “to” and grafting “from”. The grafting “to” methodologyinvolves the reaction of an end-functionalized polymer chain with anappropriate surface to anchor the polymer. Although grafting “to” allowsfor full characterization of the polymer before grafting, it is onlyapplicable to limited substrates, requires terminal functionality on thepolymer chain-end, and the grafting efficiency decreases with increasingmolecular weight.

Grafting “from” overcomes some of these limitations and has been usedwith a variety of polymerization techniques. By anchoring a suitableinitiator to the substrate, polymer chains can be grown directly by theuse of these various polymerization techniques. The majority ofsurface-anchored initiators involve the formation of a self-assembledmonolayer (SAM) on an appropriate substrate. However, SAMs have limitedstability to various reagents and are not substrate-independent.

Surface-initiated atom transfer radical polymerization (SI-ATRP) hasbecome the workhorse in the grafting “from” literature due to the easein polymerizing a wide variety of monomers containing an array offunctional groups with a high degree of control. Control in ATRP comesfrom the reversible redox activation of a dormant polymer chain-end(halide functionalized) by a halogen transfer to a transition metalcomplex. Many parameters are involved which can be tuned for bettercontrol, which provides an impressive window in which well-controlledpolymers of numerous different monomers can be synthesized.

While the most common method for anchoring ATRP initiators to thesubstrate involves the formation of a SAM, some alternative methods havebeen presented in the literature. von Werne et al. describe theinclusion of 10˜20% ATRP inimer in a mixture of curable monomerssuitable for photopolymerization. (See, von Werne, T. A.; Germack, D.S.; Hagberg, E. C.; Sheares, V. V.; Hawker, C. J.; Carter, K. R., AVersatile Method for Tuning the Chemistry and Size of NanoscopicFeatures by Living Free Radical Polymerization. J. Am. Chem. Soc. 2003,125, 3831-3838.) This work was further extended by the use of anacid-cleavable ATRP inimer, allowing for direct measurement of surfacegrown brushes and their comparison with polymer grown from sacrificialinitiator in solution. (See, Koylu, D.; Carter, K. R.,Stimuli-Responsive Surfaces Utilizing Cleavable Polymer Brush Layers.Macromolecules 2009, 42, 8655-8660.) An alternate method for creating aninimer layer is to form an adhesive coating which contains moieties forinitiator incorporation. For example, layers of poly(allylamine)(deposited by pulsed plasma polymerization) or catechol-amine (depositedby solution incubation) on various substrates were used forfunctionalizing the surface with ATRP initiators. (See, Yameen, B.;Khan, H. U.; Knoll, W.; Förch, R.; Jonas, U., Surface InitiatedPolymerization on Pulsed Plasma Deposited Polyallylamine: A PolymerSubstrate-Independent Strategy to Soft Surfaces with Polymer Brushes.Macromol. Rapid Commun. 2011, 32, 1735-1740, Coad, B. R.; Lua, Y.;Meagher, L., A Substrate-Independent Method for Surface Grafting PolymerLayers by Atom Transfer Radical Polymerization: Reduction of ProteinAdsorption. Acta Biomaterialia 2012, 8, 608-618, Fan, X.; Lin, L.;Dalsin, J. L.; Messersmith, P. B., Biomimetic Anchor forSurface-Initiated Polymerization from Metal Substrates. J. Am. Chem.Soc. 2005, 127, 15843-15847.) More recently, a catechol-functionalizedmethacrylamide and a methacrylate ATRP inimer were copolymerized by freeradical polymerization followed by deposition on Ti substrates forpolymer brush growth. (See, Wang, X.; Ye, Q.; Gao, T.; Liu, J.; Zhou,F., Self-Assembly of Catecholic Macroinitiator on Various Substrates andSurface-Initiated Polymerization. Langmuir 2012, 28, 2574-2581.)

SUMMARY

Crosslinkable random copolymers and crosslinked copolymer films formedfrom the copolymers are provided. The copolymers and copolymer filmscomprise functional groups that are ATRP initiators. Methods for formingthe crosslinked films and for using the crosslinked films for polymerbrush growth and for the self-assembly of block copolymers films arealso provided.

One embodiment of a random copolymer comprises a copolymer of a firstmonomer comprising an alkyl halide functional group that is capable ofacting as an ATRP initiator and a second monomer comprising acrosslinkable functional group, wherein alkyl halide functional groupsand the crosslinkable functional groups are pendant groups on thebackbone of the random copolymer. The first monomer may be selected fromacrylate monomers having an alkyl halide functional group, methacrylatemonomers having an alkyl halide functional group, styrene monomershaving an alkyl halide functional group, and combinations thereof

One embodiment of a crosslinked copolymer film comprises crosslinkedrandom copolymer chains, wherein the random copolymer is a copolymer ofa first monomer comprising an alkyl halide functional group that iscapable of acting as an ATRP initiator, and a second monomer, andfurther wherein the alkyl halide functional groups are pendant groups onthe backbone of the random copolymer chains and the crosslinks betweenthe random copolymer chains are formed between crosslinkable functionalgroups on the second monomers.

One embodiment of a method of making a crosslinked copolymer filmcomprises the steps of: depositing a film of a random copolymer on asubstrate surface, wherein the random copolymer is a copolymer of afirst monomer comprising an alkyl halide functional group that iscapable of acting as an ATRP initiator and a second monomer comprising acrosslinkable functional group, and further wherein the alkyl halidefunctional groups and the crosslinkable groups are pendant groups on thebackbone of the random copolymer chains; and subsequently crosslinkingthe crosslinkable functional groups, such that crosslinks are formedbetween the random copolymer chains in the film.

One embodiment of a method of making a polymer brush using thecrosslinked copolymer films comprises the steps of: exposing thecopolymer film to a solution comprising polymerizable monomers and atransition metal complex under reaction conditions in which the halidesof the copolymer film initiate the polymerization of the polymerizablemonomers via atom transfer radical polymerization.

One embodiment of a method of forming a self-assembled block copolymerfilm comprises the steps of depositing a block copolymer film on apatterned substrate, wherein the patterned substrate induces the blockcopolymer to self-assemble into patterned domains. The patternedsubstrate comprises a first set of surface regions and a second set ofsurface regions. The first set of surface regions comprises a polymerbrush disposed on a crosslinked copolymer film, the film comprisingcrosslinked random copolymer chains, wherein the random copolymer is acopolymer of a first monomer comprising an alkyl halide functional groupand a second monomer, and further wherein the alkyl halide functionalgroups are pendant groups on the backbone of the random copolymer chainsin the film and the crosslinks between the random copolymer chains inthe film are formed between crosslinkable functional groups on thesecond monomers.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1. (A) Structures of some styrene-based inimers; and (B) structuresof some (meth)acrylate-based inimers.

FIG. 2. (A) Structures of some (meth)acrylate-based crosslinkablemonomers; and (B) structures of some styrene-based crosslinkablemonomers.

FIG. 3 is a schematic illustration of the process for the formation of abrush polymer using a crosslinked ATRP initiating polymer film.

FIG. 4 is a schematic illustration of the process for the formation of aself-assembled block copolymer film.

FIG. 5. Structure of (A) ATRP initiator ethyl 2-bromoisobutyrate, (B)initiating sites on the surface, and (C) the synthetic scheme for theinitiator p-(2-bromoisobutyloylmethyl)ethylbenzene.

FIG. 6. Plots showing (A) M_(n) and PDI of the PMMA grown in solution asa function of polymerization time during SI-ATRP of MMA, and (B) thelinear increase in brush thickness as a function of time.

FIG. 7. Kinetics data from SI-ATRP of MMA showing (A) a linearcorrelation between the brush thickness and the M_(n) of the PMMA grownin solution, and (B) the uniform average chain density at variouspolymerization times.

FIG. 8. Static water contact angles for various substrates before inimermat formation, after cross-linking and after growth of PMMA brushes.

DETAILED DESCRIPTION

Crosslinkable random copolymers comprising ATRP initiators andcrosslinked copolymer films formed from the copolymers are provided.Also provided are methods of synthesizing the random copolymers and thefilms. The crosslinked films may be used for polymer brush growth viaSI-ATRP and the resulting polymer brushes may be patterned and used assubstrates for the self-assembly of block copolymers films.

The random copolymers, which are polymerized from one or more alkylhalide functional inimers and one or more monomers having acrosslinkable functionality, are characterized by pendant ATRPinitiating groups and pendant crosslinkable groups. By adjusting theratio of inimer to crosslinkable monomer and/or by polymerizingadditional comonomers in the random copolymers, the inimer content ofthe copolymers can be varied over a broad range. Thus, the copolymerscan be synthesized with very high halogen atom contents. When suchcopolymers are crosslinked into a copolymer film, the films are suitedfor use as grafting substrates for densely grown polymer brushes. Thecrosslinked films are stable on a broad ranges of substrate materials,even in the absence of covalent bonding to the substrate and in thepresence of destabilizing organic solvents.

The inimers used to synthesize the copolymers comprise an ATRPinitiating group and a monomer fragment. The ATRP initiating groupscomprise alkyl halide groups, including secondary and tertiary alkylhalides. The monomer fragment is a portion of the inimer having afunctional group that is polymerized into the copolymer backbone chain.Suitable monomer fragments include acrylates and methacrylates(collectively “(meth)acrylates”) and styrenes. Thus, the inimersgenerally comprise alkyl halide group-containing ethylenicallyunsaturated monomers. P-(2-bromoisobutyloylmethyl)styrene (BiBMS) is oneexample of a suitable inimer. The structure of this inimer, along withthe structures and names of other suitable styrene-based inimers, isshown in FIG. 1(A). The structures and names of some suitable(meth)acrylate-based inimers are shown in FIG. 1(B).

The crosslinkable monomers employed as comonomers in the polymersynthesis comprise a crosslinkable functional group and a monomerfragment. The crosslinkable functional groups may be thermallycrosslinkable groups (i.e., wherein crosslinking is induced by heating)or photocrosslinkable groups (i.e., wherein crosslinking is induced byradiation), such as UV-crosslinkable groups. However, becauseradiations, such as UV radiation, can remove the halogen atoms from theinitiating groups, thermally crosslinkable inimers may be preferred.Epoxy groups are an example of a suitable crosslinkable group. Like themonomer fragment of the inimer, the monomer fragment of thecrosslinkable monomer may comprise a styrene group or a (meth)acrylategroup. One example of a suitable self-crosslinkable monomer is glycidylmethacrylate (GMA). The structure of GMA, along with the structures ofother suitable (meth)acrylate-based crosslinkable monomers, is shown inFIG. 2(A). The structures of some suitable styrene-based crosslinkablemonomers—including a monomer comprising a cycloaliphatic unit based onstyrene, are shown in FIG. 2(B). Other crosslinkable epoxygroup-containing ethylenically unsaturated monomers can also be used ascomonomers in the polymerization, including other aliphatic or bicyclicepoxides. Examples of such epoxy group-containing ethylenicallyunsaturated monomers can be found in U.S. Pat. No. 7,317,055.

By way of illustration, a random copolymer may be copolymerized fromBiBMS as the inimer and GMA as the crosslinkable monomer. The synthesisof such a copolymer is described in detail in the example below and thestructure of the resulting copolymer chain is shown in panel (A) of FIG.3.

Optionally, additional comonomers that are neither inimers norcrosslinkable can be polymerized into the random copolymers in order toalter the properties of the copolymer or to adjust the inimer and/orcrosslinker density along the polymer chain. Examples of additionalcomonomers include (meth)acrylates and vinyl monomers, such as styrenes.

The optimal monomer content of the random copolymers, which can bemeasured via ¹H NMR spectroscopy, will depend on the intended finalapplication for the copolymer. For example, if the random copolymerswill be used as ATRP initiators for grafting polymer brushes, the inimercontent will depend on the desired grafting density. By way ofillustration, some embodiments of the random copolymers will have aninimer content in the range from about 10% to about 99%. This includesembodiments in which the inimer content is in the range from about 20%to about 96% and further includes embodiments in which the inimercontent is in the range from about 80% to about 98%. In someembodiments, the random copolymer has an inimer content of at least 40%,at least 60%, at least 80% or at least 90%.

The random copolymers desirably include enough of the crosslinkablemonomer to provide a stable crosslinked thin film. Typically, this canbe accomplished by including about 1% to about 30% (e.g., from about 2%to about 20% or about 4% to about 16%) crosslinkable monomer in thecopolymer. However, percentages outside of these ranges can also beused.

The random copolymers can be synthesized using conventionalpolymerization techniques, including free-radical polymerization orreversible addition fragmentation chain-transfer (RAFT) polymerizationtechniques.

Once the random copolymers have been formed, they can be crosslinkedinto polymeric films bearing pendant ATRP initiating groups by theapplication of heat and/or light. This is illustrated schematically inpanel (B) of FIG. 3. By synthesizing the random copolymers beforecrosslinking them, problems related to blend immiscibility can beavoided, ensuring the fabrication of a highly homogeneoussingle-component polymer film. Thus, the present methods can bedistinguished from methods of making crosslinked polymer films in whicha monomer solution is prepared and copolymerization and crosslinking ofthe monomers occur simultaneously. Because polymer blend immiscibilitycan be an issue in such methods, the resulting crosslinked films maysuffer from inhomogeneity and, therefore, may be multi-componentsystems.

The crosslinking can be carried out on a substrate surface by spreadingthe random copolymers on the surface using, for example, spin-coatingtechniques and then inducing the crosslinking reactions. For example, ifthe random copolymers comprise thermally crosslinkable pendant groups,crosslinking may be induced by subjecting the copolymers to a thermalanneal. Enough of the copolymer can be deposited onto the substratesurface to achieve a desired thickness for the crosslinked film. Theoptimal thickness of the film will depend on the desired application ofthe film. For example, if the crosslinked random copolymer film is to beused as a substrate for pattern transfer in a block copolymer filmapplication, a thinner film may be desirable, such as a film having athickness in the range from about 2 nm to about 6 nm. However,crosslinked films having thicknesses outside of this range may befabricated. For example, the films may have a thickness in the rangefrom about 2 to about 100 nm. This includes films having a thickness inthe range from about 2 to about 30 nm and further includes films havinga thickness in the range from about 10 to about 30 nm. Generally, verythin films (e.g., those having a thickness of about 6 nm or less) willbenefit from a higher crosslinking density in order to improve theirstability against delamination from the surface of the underlyingsubstrate.

The films can be crosslinked on a variety of substrates and do not needto form covalent bonds with the substrates to achieve stability againstdelamination. Thus, although covalent bonds may be formed between therandom copolymers and the underlying substrate in some instances (as inthe case where the substrate comprises an oxide), in some embodimentsthere is no covalent bonding between the crosslinked films and theunderlying substrate. Examples of substrate materials on which thecrosslinked films may be formed include silicon, metals (e.g., noblemetals, such as gold and platinum), glass, indium-tin-oxide (ITO) coatedglass and magnesium oxide. The substrate surfaces upon which the filmsare formed may be planar or non-planar surfaces.

Although some of the ATRP initiating halogen atoms may be lost duringthe crosslinking process, crosslinked films made from random copolymershaving a high ATRP initiator content will themselves have a high ATRPinitiator content. Some embodiments of the crosslinked films have ahalogen atom density (e.g., a bromine atom density) of at least 1.5halogen atoms/nm³. This includes films having a halogen atom density ofat least 1.8 halogen atoms/nm³. These ATRP initiator content valuesrefer to values based on the halogen atom density of the film,determined using XPS, as described in the example below.

Once the crosslinked random copolymer films have been prepared, they canbe used as grafting substrates for SI-ATRP growth of polymer andcopolymer brushes. This process is shown schematically in panel (C) ofFIG. 3. During SI-ATRP, the crosslinked copolymer film is exposed to asolution comprising polymerizable monomers and a transition metalcomplex catalyst under reaction conditions in which the halides of thecopolymer film initiate the polymerization of the polymerizable monomersvia ATRP. In the initiation step of the ATRP process, the transitionmetal catalyst abstracts a halogen atom from an alkyl halide functionalgroup on the crosslinked film, creating a radical that is able to add toa polymerizable monomer from the solution. This creates another radicalspecies that is able to propagate the radical polymerization process.The result is a polymer brush comprising an assembly of a polymerchains, each of which is attached at one end to the crosslinked randomcopolymer film. The polymer chains of the polymer brush arecharacterized by well-defined molecular weights and low polydispersityindices (e.g., M_(w)/M_(n)≦1.5). The polymer chains of the brush may belinear, branched or hyperbranched.

Monomers that can be polymerized or copolymerized into polymer brushesvia SI-ATRP include, vinyl monomer, such as styrenes, acrylates andmethacrylates, and combinations thereof. The monomers can befunctionalized or unfunctionalized. A description of various monomersthat can be polymerized via SI-ATRP can be found in Coessens et al.,Functional Polymers by Atom Transfer Radical Polymerization, Prog.Polym. Sci. 26 (2001) 337-377.

Because the polymer brushes can be surface grafted from crosslinkedfilms having a high activated halogen atom density, the brushes canthemselves have a correspondingly high grafting density. For example,some embodiments of the polymer brushes have a grafting density of atleast 0.5 brush polymer chains per nm². This includes embodiments of thepolymer brushes having a grafting density of at least 0.8 brush polymerchains per nm². These grafting densities refer to densities calculatedfrom the dry brush thickness and the number average molecular weight(M_(n)) of the polymers, as described in the example below.

For certain applications, it may be useful to pattern the crosslinkedrandom copolymer films prior to polymer brush growth by removing one ofmore portions of the film. For example, patterned crosslinked films areuseful substrates for the formation of patterned, self-assembled blockcopolymer (BCP) films. FIG. 4 is a schematic diagram illustrating theuse of a patterned crosslinked random copolymer film in the fabricationof a patterned polymer brush layer and the subsequent fabrication of aself-assembled BCP layer. As shown in panels (A) and (B) of FIG. 4, thefirst step of the process is the formation of a patterned in thecrosslinked random copolymer film 302. As shown here, a pattern can beformed by the selective removal of the ATRP initiating halides fromselected regions 304 of the film. Such selective removal can beaccomplished using a photomask and UV irradiation. Alternatively,patterning can be accomplished by the removal of portions of the filmfrom the substrate 303. Optionally, a different material can then begrown on substrate 303 over the areas where the crosslinked film hasbeen removed. The patterning can be carried out using standardlithographic techniques, such as x-ray lithography, extreme ultravioletlithography and/or electron beam lithography. The result is a layercomprising a plurality of regions 306 of ATRP-initiating, crosslinkedrandom copolymer film interspersed with a plurality of regions 308 of amaterial 304 that is not an ATRP initiator. As shown in panel (B), thepattern can be a regular repeating pattern, such as a series ofalternating stripes aligned along their longitudinal axes. However,other patterns, including random patterns, can also be used. If thepatterned film is to be used in the fabrication of a self-assembled BCPlayer, material 304 should be selected such that it is preferentiallywet by one of the polymer blocks in the BCP.

As illustrated in panel (C) of FIG. 4, a polymer brush 310 can beselectively grafted from surface regions 306, which comprise the ATRPinitiating crosslinked polymer film, to provide a patterned polymerbrush layer. A layer of a BCP 312 is then deposited over the patternedpolymer brush layer (panel (D)), which self-assembles (typically uponthe application of a thermal anneal) into a plurality of domains 314,316 (panel (E)). Although not shown here, the process can also includethe subsequent steps of selectively removing one of the polymer domainsfrom BCP layer 312 to form a mask and transferring the mask pattern intothe underlying substrate.

The block copolymer film, which typically comprises a diblock copolymer,defines a pattern, which is produced by the self-assembly of the blockcopolymer induced by the underlying crosslinked copolymer film. Thecharacteristics of the pattern may vary. In some embodiments, thepattern comprises domains oriented perpendicular with respect to thesurface of the underlying substrate. In other embodiments, the patterncomprises domains oriented parallel with respect to the surface of theunderlying substrate. The domains themselves may comprise a variety ofmolecular structures. In some embodiments, the domains comprisecylinders. In other embodiments, the domains comprise lamellae. Finally,by “perpendicular” it is meant that the molecular structures within thedomains form an approximate, but not necessarily exact, right angle withthe surface of the substrate. Similarly, the term “parallel” is meant toencompass molecular structures which are oriented approximately parallelto the surface of the substrate.

The block copolymer may be formed from a variety of copolymers. In someembodiments, the block copolymer comprises a diblock copolymer of apolystyrene (PS) and polymethyl methacrylate (PMMA).

Example Experimental Section

Materials.

All chemicals were purchased from Sigma-Aldrich and used without furtherpurification unless otherwise noted. 4-vinylbenzyl alcohol wassynthesized according to a literature procedure. (See, Zhao, L. J.;Kwong, C. K. W.; Shi, M.; Toy, P. H., Optimization ofPolystyrene-Supported Triphenylphosphine Catalysts foraza-Morita-Baylis-Hillman Reactions. Tetrahedron 2005, 61 (51),12026-12032.) Copper(I) bromide (99.999%) was stirred in acetic acidovernight, suction-filtered, washed with ethanol and then dried undervacuum. Styrene, glycidyl methacrylate and methyl methacrylate (MMA)were stirred over calcium hydride and then distilled under vacuum.4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid wassynthesized according to a literature procedure. (See, Moad, G.; Chong,Y. K.; Postma, A.; Rizzardo, E.; Thang, S. H., Advances in RAFTPolymerization: the Synthesis of Polymers with Defined End-Groups.Polymer 2005, 46, 8458-8468.) 2,2′-azobis(2-methylpropionitrile) (AIBN)was recrystallized from acetone and dried under vacuum.

Synthesis of p-(2-bromoisobutyloylmethyl)styrene (BiBMS)

4-vinylbenzyl alcohol (28.18 g, 210 mmol) was dissolved in 750 mL ofdichloromethane and chilled via an external ice bath under a nitrogenatmosphere. 4-(dimethylamino)pyridine (1.28 g, 10.5 mmol) andtriethylamine (38.47 g, 380 mmol) were added to the solution and stirreduntil dissolved. 2-bromoisobutyryl bromide (59.5 g, 259 mmol) was addedslowly by syringe and the reaction stirred at 0° C. until thin-layerchromatography (TLC) showed complete conversion. The reaction wasquenched via the addition of water and the layers separated using aseparatory funnel. The organic layer was washed twice with water, 1 MHCl and 1 M NaOH, then dried over sodium sulfate and the solvent wasremoved by rotary evaporation. The resulting oil was distilled underreduced pressure (approx. 1 torr) at 130° C. to give a near colorlessoil with a yield of 50 g (84% of theoretical yield). ¹H NMR (400 MHz,CDCl₃) δ 7.41 (d, J=8.3 Hz, 2H), 7.33 (d, J=8.3 Hz, 2H), 6.71 (dd,J=17.6, 10.9 Hz, 1H), 5.76 (dd, J=17.6, 0.9 Hz, 1H), 5.27 (dd, J=17.6,0.9 Hz, 1H), 5.19 (s, 2H), 1.94 (s, 6H). ¹³C NMR (101 MHz, CDCl₃) δ171.49, 137.71, 136.33, 134.89, 128.24, 126.43, 114.44, 67.38, 55.72,30.81.

Synthesis of p-(2-bromoisobutyloylmethyl)ethylbenzene

4-ethylbenzyl alcohol (5.0 g, 36.7 mmol) was dissolved in 100 mLdichloromethane with 4-(dimethylamino)pyridine (22.4 mg, 0.18 mmol) andtriethylamine (6.72 g, 66 mmol) under nitrogen and chilled to 0° C.2-bromoisobutyryl bromide (10.4 g, 45.2 mmol) was added slowly bysyringe and the reaction stirred at 0° C. until TLC showed completeconversion. The reaction was quenched via the addition of water and thelayers separated using a separation funnel. The organic layer was washedtwice with water, 1 M HCl and 1 M NaOH, then dried over sodium sulfateand the solvent was removed by rotary evaporation. The resulting oil wasthen passed through a column of basic alumina to yield 9 g (86% yield).¹H NMR (400 MHz, CDCl₃) δ 7.30 (d, J=8.3 Hz, 2H), 7.20 (d, J=8.3 Hz,2H), 5.17 (s, 2H), 2.65 (q, J=7.6 Hz, 2H), 1.94 (s, 6H), 1.24 (t, J=7.6Hz, 3H).

Synthesis of P(BiBMS-r-GMA).

BiBMS (2.55 g, 9 mmol), GMA (0.14 g, 1 mmol),4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (40.3mg, 0.1 mmol), and AIBN (4.1 mg, 0.025 mmol) were added to 1 gramanisole in a 10 mL Schlenk flask equipped with a magnetic stir bar. Themixture was degassed via three freeze-pump-thaw cycles and placed in anoil bath at 85° C. for 16 hours. The polymerization was quenched bycooling the flask with cold water and exposure to air. The resultingviscous oil was then diluted with THF and precipitated into hexanes. Thepolymer was collected as a yellow powder and dried under vacuum. Therelative composition of the two monomers was determined by ¹H NMRspectroscopy.

Synthesis of P(S-r-BiBMS-r-GMA)

BiBMS (0.849 g, 3 mmol), GMA (0.071 g, 0.5 mmol), styrene (0.68 g, 6.5mmol), 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid(40.3 mg, 0.1 mmol), and AIBN (4.1 mg, 0.025 mmol) were added to 1 gramanisole in a 10 mL Schlenk flask equipped with a magnetic stir bar. Themixture was degassed via three freeze-pump-thaw cycles and placed in anoil bath at 75° C. for 16 hours. The polymerization was quenched bycooling the flask with cold water and exposure to air. The resultingviscous oil was then diluted with THF and precipitated into hexanes. Thepolymer was collected as a yellow powder and dried under vacuum. Therelative composition of the three monomers was determined by ¹H NMRspectroscopy.

Substrate Preparation and Thin Film Formation.

A solution of P(BiBMS-r-GMA) or P(S-r-BiBMS-r-GMA) (0.3% w/w) in toluenewas spin-coated onto silicon wafers that had been cleaned using piranhaacid (7:3 H₂SO₄:H₂O₂, caution: reacts violently with organic compounds).The substrate was then annealed under vacuum at 200° C. for varioustimes to produce a cross-linked thin film. After annealing, thesubstrate was soaked in toluene and rinsed copiously with fresh tolueneto remove uncross-linked polymer.

Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP).

MMA (4 grams, 40 mmol), tris-(2-pyridylmethyl)amine (60 mg, 0.21 mmol),CuBr (4.0 mg, 0.028 mmol), CuBr₂ (6.4 mg, 0.028 mmol),p-(2-bromoisobutyloylmethyl)ethylbenzene (7.6 mg, 0.027 mmol) andanisole (40 g) were mixed and sonicated until the copper completelydissolved into the brown solution. Equal amounts of the polymerizationmixture were then added to four different flasks containing substratescovered with the cross-linked inimer mat and magnetic stir bars. Theflasks were then degassed via three freeze-pump-thaw cycles. Afterwarming to room temperature using a water bath, the flasks were immersedin a 60° C. oil bath for the requisite amount of time. After therequisite amount of time elapsed, the flasks were cooled to roomtemperature using running water and the flask opened to atmosphere. Thesubstrate was then removed from the flask and washed copiously with THFand water. After washing, the substrate was further soaked in THF for 2hours, then sonicated in THF for 10 minutes, followed by rinsing withTHF to remove any ungrafted polymer chains and dried using a stream ofnitrogen. Some samples were further subjected to Soxhlet extractionusing acetone for 18 hours to ensure that the THF treatment removed anyungrafted polymer.

Characterization.

¹H NMR and ¹³C NMR spectra were recorded in CDCl₃ using a BrukerAvance-400 spectrometer with TMS as internal standard. Gel-permeationchromatography (GPC) was performed using a Viscotek 2210 system equippedwith three Waters columns (HR 4, HR 4E, HR 3) and a 1 mL/min flow rateof THF as eluent at 30° C. Monodisperse PS standards were used forcalibration. The film thickness of the samples was measured byellipsometry (Rudolph Research Auto EL). The surface topography of thecross-linked mat and brush layer was examined using a Nanoscope IIIMultimode atomic force microscope (Digital Instruments) in tapping mode.Thermal gravimetric analysis was performed on a TA Instruments Q500using a heating rate of 10° C. per minute under a nitrogen atmosphere.X-ray photoelectron spectroscopy (XPS) was done on a PerkinElmer 5400ESCA spectrometer Phi model using a Mg X-ray source (300 W, 15 kV) at atakeoff angle of 45° from the substrate normal. The hemispherical energyanalyzer was used in hybrid mode with a 1 mm×3.5 mm area aperture.Survey spectra were collected at pass energy of 89.45 eV with a scanstep size of 1.0 eV. High-resolution multiplex spectra were collectedwith pass energy of 35.75 eV and a step size of 0.05 eV. Water contactangle was measured with a Dataphysics OCA 15 Plus.

Results and Discussion

Synthesis of Random Copolymer.

The synthesis of the random copolymer from BiBMS and GMA by RAFTpolymerization, where the inimer is the majority component of thecopolymer and GMA allows for the formation of a cross-linked mat, isshown in Scheme 1 of FIG. 3. The composition of the copolymer wasdetermined to be 84% BiBMS and 16% GMA by ¹H NMR spectroscopy.

Formation of Cross-linked mat and XPS quantification. TGA shows aninitial decomposition step at 230° C., likely due to decarboxylation andsubsequent loss of propylene through bromide elimination. Therefore, across-linking temperature of 200° C. was chosen to avoid unintentionaldegradation of the polymer during cross-linking while maintaining arapid cross-linking rate. A 0.3% w/w solution in toluene of randomcopolymer was spin-coated onto silicon wafers to yield ˜9 nm thickfilms. The substrates were then heated under vacuum at 200° C. forpredetermined times and quenched to room temperature. After repeatedwashing in toluene, the samples were dried under nitrogen and theremaining thickness measured by ellipsometry. The film thickness aftercross-linking but prior to washing was used as the initial thickness fordetermining cross-linking efficiency.

After 10 minutes, the normalized thickness started to plateau atapproximately 90% of the original value. Despite the relatively lowmolecular weight (11.8 kDa), extensive cross-linking was achievedthrough a comparatively large amount of GMA. To determine the effect ofthis processing on the initiating units, the amount of bromine in themat was quantified by XPS. XPS survey spectra showed the presence ofbromine. The film thickness was controlled to be less than the typicalXPS sampling depth (˜10 nm) so that Si peaks from the substrate werealso visible.

The absolute elemental concentration can be determined from theintensity ratio of the element in the organic thin film to an element inthe underlying substrate whose atomic density is known. (See,Paoprasert, P.; Spalenka, J. W.; Peterson, D. L.; Ruther, R. E.; Hamers,R. J.; Evans, P. G.; Gopalan, P., Grafting of Poly(3-hexylthiophene)Brushes on Oxides Using Click Chemistry. J. Mat. Chem. 2010, 20,2651-2658, Franking, R. A.; Landis, E. C.; Hamers, R. J., Highly StableMolecular Layers on Nanocrystalline Anatase TiO₂ through PhotochemicalGrafting. Langmuir 2009, 25, 10676-10684.) The native oxide layer wasemployed as a reference, as the density of silicon in the oxide is wellknown and the oxide thickness can be determined experimentally.

In general, the intensity of the XPS signal is given by the equation

$\begin{matrix}{I_{s} = {I_{0} \cdot ^{- \frac{d}{\lambda cos\theta}}}} & (1)\end{matrix}$

where I_(S) is the detected intensity, I₀ is the incident electronintensity, d is the depth of the electrons emitted in the material, λ isthe inelastic mean free path (IMFP) of electrons in the material and θis the take-off angle. In the sample, signal from emitted electrons inthree different layers contribute to the XPS spectrum: emitted electronsfrom silicon atoms in the silicon substrate, from silicon atoms in thenative oxide and from various elements (C, O and Br) in the cross-linkedmat. The intensity of emitted electrons from silicon atoms in the nativeoxide, I_(Si,SiO) ₂ can be calculated by integrating the exponentialequation

$\begin{matrix}{I_{{Si},{SiO}_{2}} = {N_{{Si},{SiO}_{2}} \cdot {\int\limits_{0}^{T_{ox}}{^{- \frac{z}{\lambda_{{Si},{{{SiO}_{2} \cdot \cos}\; \theta}}}}{z}}}}} & (2)\end{matrix}$

through the native oxide layer, typically 1.1 nm in the experiments,where N_(Si,SiO) ₂ is the number density of silicon atoms in nativeoxide layer (22.06 atoms/nm³), T_(ox) is the thickness of the nativeoxide, and λ_(Si,SiO) ₂ is the IMFP of the electrons from silicon in thenative oxide layer. The emitted electrons further travel through theorganic thin film layer, and then escape from the material and reach thedetector, resulting in the Si(2p) peak at 101.5˜104.5 eV. Therefore, theintensity of the peak is:

$\begin{matrix}{\frac{A_{Si}}{S_{Si}} = {N_{{Si},{SiO}_{2}} \cdot ^{- \frac{L}{{\lambda_{{Si},{organic}} \cdot \cos}\; \theta}} \cdot {\int\limits_{0}^{T_{ox}}{^{- \frac{z}{{\lambda_{{Si},{SiO}_{2}} \cdot \cos}\; \theta}}{z}}}}} & (3)\end{matrix}$

where A_(Si) is the integrated intensity of the Si(2p) peak of SiO₂,S_(Si) is the relative sensitivity factor of Si(2p), L is the thicknessof the organic thin film and λ_(Si,organic) is the IMFP of the electronsemitted from silicon traversing the organic thin film layer.

The electrons emitted from the organic layer, specifically from thebromine atoms in the cross-linked mat, travel through the organic layerand reach the detector, resulting in a Br(3d) peak at 69.0˜72.5 eV.Thus, the intensity of the Br(3d) peak can be formulated by the equationof

$\begin{matrix}{\frac{A_{Br}}{S_{Br}} = {N_{{Br},{organic}} \cdot {\int\limits_{0}^{L}{^{- \frac{z}{{\lambda_{{Br},{organic}} \cdot \cos}\; \theta}}{z}}}}} & (4)\end{matrix}$

where A_(Br) is the integrated intensity of Br(3d) peak, S_(Br) is therelative sensitivity factor of Br(3d) and λ_(Br,organic) is the IMFP ofthe electrons from bromine in the organic thin film layer. The intensityratio of Si(2p) to Br(3d) is given by

$\begin{matrix}{\frac{\frac{A_{Si}}{S_{Si}}}{\frac{A_{Br}}{S_{Br}}} = \frac{N_{{Si},{SiO}_{2}} \cdot ^{- \frac{L}{{\lambda_{{Si},{organic}} \cdot \cos}\; \theta}} \cdot {\int\limits_{0}^{T_{ox}}{^{- \frac{z}{{\lambda_{{Si},{SiO}_{2}} \cdot \cos}\; \theta}}{z}}}}{N_{{Br},{organic}} \cdot {\int\limits_{0}^{L}{^{- \frac{z}{{\lambda_{{Br},{organic}} \cdot \cos}\; \theta}}{z}}}}} & (5)\end{matrix}$

from equation (3) and (4). Equation (5) can be rewritten as

$\begin{matrix}{N_{{Br},{organic}} = {\frac{\frac{A_{Br}}{S_{Br}}}{\frac{A_{Si}}{S_{Si}}} \cdot \frac{N_{{Si},{SiO}_{2}} \cdot ^{- \frac{L}{{\lambda_{{Si},{organic}} \cdot \cos}\; \theta}} \cdot {\int\limits_{0}^{T_{ox}}{^{- \frac{z}{{\lambda_{{Si},{SiO}_{2}} \cdot \cos}\; \theta}}{z}}}}{\int\limits_{0}^{L}{^{- \frac{z}{{\lambda_{{Br},{organic}} \cdot \cos}\; \theta}}{z}}}}} & (6)\end{matrix}$

where, N_(Br,organic) is the number density of bromine in thecross-linked mat.

The thickness of the cross-linked mat was 5.8±0.4 nm, and λ_(Si,SiO) ₂ ,λ_(Si,organic) and λ_(Br,organic) values were obtained from theliterature. (See, Powell, C. J.; Jablonski, A., Evaluation of ElectronInelastic Mean Free Paths for Selected Elements and Compounds. SurfInterface Anal. 2000, 29, 108-114, Laibinis, P. E.; Bain, C. D.;Whitesides, G. M., Attenuation of Photoelectrons in Monolayers ofNormal-Alkanethiols Adsorbed on Copper, Silver, and Gold. J. Phys. Chem.1991, 95, 7017-7021.) From the high resolution XPS spectra of Br(3d) andSi(2p), the integrated intensity ratio, corrected for the relativesensitivity factors of Br(3d) and Si(2p) (S_(Br)=1.053 andS_(Si)=0.339), was found to be 2.23±0.26[(A_(Br)/S_(Br))/(A_(Si)/S_(Si)) in equation (6)]. From this result, thecalculated density of bromine in the cross-linked N_(Br,organic), was1.86±0.12 Br atoms/nm³. For comparison, the theoretical amount ofinitiator per unit volume (π_(ini)) calculated using the number averagemolecular weight (M_(n)) and the number of inimer units per chain (thesame as N_(Br)), as determined from GPC and ¹H NMR, respectively can becalculated from the equation

ρ_(ini)=ρ_(p) ·N _(av) ·N _(Br) /M _(n)  (7)

where, N_(av) is Avogadro's number and ρ_(p) is the density of copolymerwhich is 1.30 g/cm³ (assuming the density of copolymer is similar to theweighted average density of the two monomers). Calculated ρ_(ini) is2.53 initiators/nm³, which is slightly higher than the experimentallydetermined value. The ratio of experimental density to calculatedabsolute initiator density is 0.735, suggesting that approximately 25%of the bromine is lost during processing.

Next the calculated results were compared with the relative initiatorconcentration by looking at the ratio of bromine to carbon. To do so, anexamination was made of the calculated intensity ratio of Br(3d) toC(1s) peaks from high resolution XPS spectra (N(Br)/N(C)) and thetheoretically estimated value from the compositional informationobtained from ¹H NMR. The experimental data gave a N(Br)/N(C) of0.0523±0.001, while the theoretical value was 0.0700, resulting in theratio of experimental value to theoretical value of 0.747. This valueobtained from the relative intensity ratio is in excellent agreementwith the values of N_(Br)/ρ_(ini) of 0.735 obtained from thequantitative comparison with the Si(2p) peak above.

In order to further verify the quantitative XPS analysis model, adifferent copolymer, which reduced the amount of inimer by incorporatingan additional monomer that does not have bromine e.g. styrene wassynthesized. This copolymer consisted of styrene, BiBMS and GMA(M_(n)=11.7 kDa, PDI=1.61, F_(st)=0.62, F_(BiBMS)=0.29, F_(GMA)=0.09,determined by GPC and ¹H NMR). The cross-linked mat (5.0±0.2 nm) wasformed in the same manner as before on native oxide/Si substrate,followed by XPS characterization. Using equation (6) and (7), theexperimental (N_(Br)) and the theoretical (ρ_(ini)) number density ofinitiator were 0.90±0.18 Br atoms/nm³ and 1.15 initiators/nm³,respectively. This, again confirmed the experimental value to be lowerthan the theoretical estimate (ratio of experimental to theoreticalvalues was 0.78, comparable to the 0.735 from the previous calculation).Furthermore, it shows that the amount of initiator on the surface can beeasily tuned by introducing a comonomer and thereby changing the molarratio of inimer in the copolymer.

Surface Initiated ATRP of PMMA.

Following the quantitative analysis of inimer in the cross-linked film,the growth of polymer brushes from the mat was examined Specifically, ofinterest was the resulting chain density of grown polymer brushesconsidering the sheer density of initiating groups available forpolymerization (1.86 Br/nm³). Therefore, a qualitative determination ofthe chain density on the surface was made by correlating the molecularweight of sacrificial polymer grown in solution to the resulting brushlayer thickness of the substrate using a well-known monomer, MMA.

In the first experiments, standard ATRP conditions were used for thesolution growth of PMMA (ethyl-2-bromoisobutyrate, CuBr and PMDETA) andthe sacrificial polymer grown in solution (PDI<1.2) was successfullycontrolled. However, under these conditions, the solution molecularweight did not reasonably correlate to the measured brush thickness. Thepolymer brush height implied a length longer than the fully-extendedcontour length of a polymer with the solution molecular weight that wasmeasured. Therefore, several parameters were optimized.

First, it needed to be ensured that the initiating species had assimilar a reactivity between solution and substrate as possible. Eventhough ethyl 2-bromoisobutyrate (Scheme 2a, FIG. 5(A)) has the sameα-haloester type ATRP initiating moiety as BiBMS (Scheme 2b, FIG. 5(B)),Matyjaszewski et al. have shown that going from methyl2-bromoisobutyrate to ethyl 2-bromoisobutyrate almost halves the ATRPequilibrium constant keeping everything else constant. (See, Tang, W.;Matyjaszewski, K., Effects of Initiator Structure on Activation RateConstants in ATRP. Macromolecules 2007, 40, 1858-1863, Tang, W.; Kwak,Y.; Braunecker, W.; Tsarevsky Nicolay, V.; Coote Michelle, L.;Matyjaszewski, K., Understanding Atom Transfer Radical Polymerization:Effect of Ligand and Initiator Structures on the Equilibrium Constants.J. Am. Chem. Soc. 2008, 130, 10702-10713.) Therefore, it was importantto account for this possible reactivity difference. A new ATRP initiatorp-(2-bromoisobutyloylmethyl)ethylbenzene (Scheme 2c, FIG. 5(C)) was thussynthesized to mimic the propagating reactive sites on the surface byhaving the same molecular structure with an ethyl moiety instead ofvinyl. Secondly, as the monomer is depleted in solution, it is likelythat any discrepancy in molecular weight between the surface and thesubstrate will be magnified due to viscosity and other kinetics issues.A very small amount of initiator (target 1500 DP) was used to initiatethe sacrificial polymer in solution which allowed the monomerconcentration to stay as constant as possible. A magnetic stir bar wasused to promote a homogeneous solution, maintaining consistentconcentrations without gradients. Also, copper (II) bromide was found tobe the most effective parameter for controlling the brush growth rate.However, by including CuBr₂, this slowed the solution growth of the PMMAto an unreasonable extent, hence a stronger ligand(tris-(2-pyridylmethyl)amine, TPMA) was used to counteract this effect.The combination of strong ligand and stoichiometrically large amount ofCuBr₂ allowed for the solution grown PMMA to have a PDI of less than1.10.

With the conditions for SI-ATRP thus optimized, kinetic studies weredone to examine the brush growth over time and its correlation tosolution molecular weight. First, the PMMA polymerized in solutionexhibits excellent control and linear kinetics as evidenced by thelinear increase in M_(n) with time (due to the low overall conversion ofmonomer) and the narrow PDI (FIG. 6). The brush thickness also exhibiteda linear relationship with polymerization time, showing awell-controlled polymerization on the surface as well. Ungrafted polymerwas removed by soaking the substrates in THF for two hours and followedby sonication for 10 minutes. To ensure this THF treatment was efficientat removing ungrafted polymer, substrates were subjected to Soxhletextraction with acetone for 18 hours. After extraction, the thicknesswas identical to the previous results from sonicating in THF, confirmingthe successful removal of ungrafted polymer, as well as the stability ofthe mat.

By correlating the dry brush thickness (h) with the M_(n) of PMMA grownin solution, the chain density (ρ) can be calculated using a standardequation (8),

ρ=h·N _(av) ·ρ/M _(n)  (8)

where ρ=1.19 g/cm³ (bulk density of PMMA). As ρ and N_(av) are constant,the relationship between brush thickness and molecular weight is linear,with the chain density as the slope of the line (equal to σ/(N_(av)·ρ)).In FIG. 7(A), the graph showing brush thickness as a function ofmolecular weight exhibits a clear linear trend over the experimentaltime examined Using equation (8), the chain density was calculated to be0.80±0.06 chains/nm². The chain density can also be calculated for eachpolymerization time by directly applying equation (8) and the resultingdata is also plotted in FIG. 7(B), demonstrating a high chain densitythat was constant for the entire polymerization time.

Atomic force microscopy (AFM) was used to examine the surface morphologyof both the inimer mat and grown polymer brush films. The AFM heightdata for the inimer mat and after 3 hours of SI-ATRP showed that inimermat exhibited extremely low roughness with an average height variationof less than 0.4 nm over a 5×5 μm² area. All polymer brush filmsexamined showed similar AFM micrographs with a roughness of 1.82 nm,3.73 nm and 3.56 nm for 1 hour, 2 hours and 3 hours of polymerization,respectively. Typically, there were large flat areas with occasionaldivots which traversed the full brush thickness down to the inimer mat.A slower polymerization rate could allow for an even more uniform film.

The effect of different substrates on the polymer brush growth was alsoexamined. In addition to the piranha treated bare Si discussedextensively, inimer mats were prepared on SiO₂/Si, Au/Si and aluminumoxide/Si substrates. SiO₂, Au and aluminum oxide substrates wereprepared by evaporation of source materials using e-beam dielectric ormetal evaporators onto Si wafers. While the bare silicon was cleanedwith piranha acid, the other substrates were not chemically cleaned topreserve their inherent properties. The inimer mat was formed in thesame manner as before, through spin-coating and thermal cross-linking.The mats were then subjected to two hours of PMMA polymerization underthe optimized conditions discussed earlier. Static water contact anglemeasurements of the bare substrate, the inimer mat and the brush layerwere taken to judge the efficacy of the film at growing brushes onvarious substrates (FIG. 8). All of the bare substrates showed theexpected contact angles while post-deposition of the mat and growth ofPMMA brushes, the contact angles for the various substrates becameequivalent.

To further confirm the growth of PMMA on the surface, XPS was used tocharacterize the brush films. The intensity ratio of deconvoluted C—C orC—H, C—O and O═C—O peaks showed good agreement with the theoreticalratios for PMMA, providing additional evidence for polymer brush growth.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”. Still further, the use of “and” or “or” is intended to include“and/or” unless specifically indicated otherwise.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A crosslinked copolymer film comprisingcrosslinked random copolymer chains, wherein the random copolymer is acopolymer of a first monomer comprising an alkyl halide functional groupthat is capable of acting as an ATRP initiator, and a second monomer,and further wherein the alkyl halide functional groups are pendantgroups on the backbone of the random copolymer chains and the crosslinksbetween the random copolymer chains are formed between crosslinkablefunctional groups on the second monomers.
 2. The copolymer film of claim1, wherein the first monomer is selected from acrylate monomers havingan alkyl halide functional group, methacrylate monomers having an alkylhalide functional group, styrene monomers having an alkyl halidefunctional group, or combinations thereof.
 3. The copolymer film ofclaim 1, wherein the crosslinkable functional groups comprise thermallyself-crosslinking functional groups.
 4. The copolymer film of claim 1,wherein the crosslinkable functional groups comprise thermallycrosslinkable epoxy groups.
 5. The copolymer film of claim 2, whereinthe crosslinkable functional groups comprise thermally crosslinkableepoxy groups.
 6. The copolymer film of claim 1, having a halogen atomdensity of at least 1.5 atoms/nm³.
 7. The copolymer film of claim 1,wherein the film is disposed on the surface of, but not covalentlybonded to, a substrate.
 8. A method of making a crosslinked copolymerfilm, the method comprising: depositing a film of a random copolymer ona substrate surface, wherein the random copolymer is a copolymer of afirst monomer comprising an alkyl halide functional group that iscapable of acting as an ATRP initiator and a second monomer comprising acrosslinkable functional group, and further wherein the alkyl halidefunctional groups and the crosslinkable groups are pendant groups on thebackbone of the random copolymer chains; and subsequently crosslinkingthe crosslinkable functional groups, such that crosslinks are formedbetween the random copolymer chains in the film.
 9. A method of making apolymer brush using the crosslinked copolymer film of claim 1, themethod comprising: exposing the copolymer film to a solution comprisingpolymerizable monomers and a transition metal complex under reactionconditions in which the halides of the copolymer film initiate thepolymerization of the polymerizable monomers via atom transfer radicalpolymerization.
 10. A method of forming a self-assembled block copolymerfilm using the crosslinked copolymer film of claim 1, the methodcomprising: patterning the crosslinked copolymer film to provide aplurality of regions comprising the alkyl halide functional groups thatare capable of acting as ATRP initiators and a plurality of regions thatdo not have ATRP initiating functional groups; exposing the crosslinkedcopolymer film to a solution comprising polymerizable monomers and atransition metal complex under reaction conditions in which the alkylhalide functional groups initiate the polymerization of thepolymerizable monomers into a polymer brush via atom transfer radicalpolymerization to provide a patterned polymer brush layer; anddepositing a block copolymer film over the patterned polymer brushlayer, wherein the patterned polymer brush layer induces the blockcopolymer to self-assemble into patterned domains.